From October 1, 1981, to the present, a total of 2,013 serological tests were performed on 462 sera. Included were 153 rat sera (660 tests), 271 mouse sera (1,225 tests) and 38 hamster sera (128 tests). Health surveillance data from NIEHS breeding colonies and from experimental animals revealed that: 1) Four different rat viruses (Sendai, PVM, KRV, and RCV/SDA), 2) Three different hamster viruses (Sendai, PVM, and SV-5), and 3) Four different mouse viruses (Sendai, PVM, MHV, and Reo-3) have been detected in animals housed in NIEHS facilities. These data suggests that MHV is widespread in our mouse rooms and that some incoming mice had seroconverted to MHV positive within 3 weeks after arrival. These mice were housed under filter tops which should limit or reduce aerosol transmission. Studies will evaluate factors which affect the distribution of infectious MHV among animals. Specifically, we will attempt to demonstrate the length of the inapparent carrier state of experimentally infected mice.